1. Field of the Invention
This invention relates to the field of electronic signal processing, and more particularly to a spectrum analyzer for providing an indication of the presence of one or multiple simultaneous radio frequency signals which may be spaced over wide frequency bands.
2. Description of the Prior Art
The presence of radio frequency signals at particular frequencies can be very usefully displayed on a spectrum analyzer. A spectrum analyzer display provides a swept time base abscissa calibrated in frequency, and an ordinate calibrated in signal amplitude. Spectrum analyzers are usefully employed in the design of signal processing circuits, are used during signal identification procedures for tactical electronic support measures, counter measures and electronic intelligence collection, etc. A spectrum analyzer thus provides a useful visual insight into the content of radio frequency band.
The derivation of the signals to display, however, has been complex and difficult, and each derivation technique suffers from various problems.
For example, according to one prior art technique, a superheterodyne receiver includes a tunable bandpass filter which scans through a frequency range, with whatever signal appearing in the pass band at a particular instant in time being displayed. Consequently the swept superheterodyne receiver cannot consider all of the frequencies at every instant in time, and it cannot provide instantaneous wideband spectrum analysis.
In order to overcome this problem and provide instantaneous wideband spectrum analysis, channellized receivers have been used. These receivers employ banks of contiguous bandpass filters and fixed tuned local oscillators. The contiguous filters encompass the frequency band to be considered. After down-converting, the resulting intermediate frequency bands are again individually divided, and the intermediate frequency bands ae down-converted to baseband which is then divided by narrow passband filters. The signal output at the narrow baseband slots which, for example, may be 12.5 megahertz wide each, resulting from a typical 1 gigahertz radio frequency band, can be scanned sequentially and displayed.
Clearly the channellized receiver introduces complex technology and high cost. Potentially wide bandwidths are displayed only at the expense of high complexity. Further, many channellized receivers utilize time sharing techniques which limit multiple simultaneous signal capability. Receivers employing frequency multiplexing, or band multiplexing, may incur ambiguity problems in the case of time coincident radio frequency signals arriving from different radio frequency bands. In addition, signal splitting and combining tend to limit the sensitivity of the receiver. This system requires a great many filters each precisely tuned, which is very expensive. The complexity, size and power consumption of this form of system results in cost and reliability problems for most applications.
In another method of providing instantaneous spectral analysis, use is made of a phase-comparison instantaneous frequency measurement receiver technique. An input signal to the receiver is split into parallel radio frequency paths, with one path containing a delay line. The two paths feed a phase detector. The output signals of the phase detector are proportional to the radio frequency amplitude and the sine and cosine of the phase difference between the two phase detector input ports. The delay line length causes a phase angle between the two signals to be proportional to the radio frequency input frequency. Thus when a pulsed radio frequency input signal is applied, simultaneous video pulses proportional to the sine and cosine of the radio frequency signal are generated. The information contained within these signals is digitized and passed to a processor or computer for generating display signals.
The phase comparison instantaneous frequency measurement receiver technique is incapable of handling multiple simultaneous signals, even though the technique is more moderate in cost and complexity than the channellized receiver technique. Since the instantaneous frequency measurement receiver depends entirely on a frequency/transfer function to derive the frequency data, it is not possible for the receiver to respond to two radio frequency voltages simultaneously. Clearly the phase detector outputs cannot assume two voltage values simultaneously.
Thus if two or more radio frequency signals are received simultaneously, an entirely erroneous measurement can be obtained. This key disadvantage to the phase comparison instantaneous frequency measurement-technique results in its susceptibility to jamming.
Another method of achieving instantaneous spectral analysis over wide radio frequency bands is to use acousto-optical processing techniques. An input signal is down-converted to an intermediate frequency band, and then amplified. The amplified signal is applied to an acoustic transducer of an acousto-optical BRAGG cell. The travelling acoustic wave launched into the optically transparent acoustic medium of the BRAGG cell creates temporary local variations in the optical refractive index. A laser beam intercepts the acoustic wave and is diffracted by an angle which is proportional to the frequency of the acoustic wave. This defracted light is then passed through an optical waveguide lens which focuses the diffracted rays onto a detector array. Each element in the array corresponds to a different acoustic frequency (and therefore radio frequency).
The acousto-optical processing system has several problems. For example, its sensitivity is degraded with narrow pulse widths, and the technology is complex, involving BRAGG cells, lasers and optical array detectors. A principal limitation of this technology is that BRAGG cells presently possess a maximum instantaneous bandwidth of approximately 1 gigahertz, since microwave signals must be down-converted to below 1 gigahertz for processing. Down-conversion by heterodyning results in a present maximum radio frequency bandwidth of approximately 1 gigahertz.